A colony formation method is known as one of experiments for examining influences of a compound on cells. This method utilizes that cells may proliferate repeatedly to form a colony if a culture environment is proper. Specifically, cells are cultured in an environment where a compound as an evaluation target is added, and how the compound affects the life and death of the cells is evaluated from a count result on the number of colonies formed under such an environment (see, for example, patent literature 1). In this method, cell colonies two-dimensionally proliferated on a surface of an agar medium or the like are generally targets to be counted.
On the other hand, a three-dimensional culture is known as a culture method for cells closer to a living body. In this case, cultured cells are called a spheroid (cell aggregation) because they become a spherical aggregation. A spheroid is formed not only from the proliferation of one cell, but also from a plurality of cells gathering together. From this, it is necessary to judge a spheroid formation ability or a spheroid maintenance ability by a plurality of cells in the observation of a spheroid.